texts: What's that ring that only now appears in black hole illustrations?

texts:
If you'll notice, space time starts to deform near the black hole, but then at considerable distance from the black hole' surface, images in the background become ultra-distorted, but then as you look even closer to the black hole, the distortion starts to look normal again almost like nothing is happening except for a minor fish-eye lens effect.

If space-time distorts in a monotonic way as you approach the surface of a black hole, how does this ring of ultra-distortion exist? Why doesn't the image look even more distorted at the surface than at that ring?


texts: Do mixed type remnant collisions produce anything interesting?

texts:
The recent detection of a binary neutron star inspiraling and colliding raises an interesting question in my mind. Type Ia supernovae are believed to be caused by white dwarf/regular star pairs and/or white dwarf pairs. Short gamma ray bursts are now known to be produced by neutron star/neutron star collisions. Black hole/black hole collisions haven't produced any verifiably detected electromagnetic radiation. What about some of the other mixed type collissions? Especially white dwarf/neutron star or neutron star/black hole?

My understanding is that stellar binaries tend to have similar mass, so mixed type remnants will likely be rather rare. Hence, why I didn't ask about white dwarf/black hole binaries. Even so, I'm curious how the large difference in density would affect things. Especially if a neutron star would be tidally shredded into an accretion disk by a black hole before they can collide, making a relatively slow event, or will the process be more rapid and violent, akin to the events we've seen so far?


texts: Can I learn astrophysics and astronomy with my basic math?

texts: I want to learn astrophysics and astronomy, I'm very interested in it because there are many questions that popped up in my head, and really curious about things, but I'm afraid of a little bit on math, but I'm a fast learner, I can understand things quickly and easily especially if it is translated on my native language, I have learned basic trigonometric functions, special angles, trigonometry, algebra, quadratic equations, and some basic math but not calculus, will proceed on that later, and I have learned basic physics, until vertical dimension, but still afraid a little on math and equations, how can I counteract that fear? I mean I really want to learn astrophysics but I'm afraid of numbers,  can you give me tips? And can you suggest me some book? Or can I learn astrophysics without interacting too much with math? Btw, I'm just taking advancement because I'm really curious and interested in astrophysics, I'm still at high school, hope you can help me.


texts: Could Sgr A* be a core of dark matter instead of a supermassive black hole?

texts:
A recent paper (related news) shows that Sgr A* at the center of the Milky Way galaxy could be a dark matter core, instead of a supermassive black hole, according to their calculation using the astrometry data of the 17 best-resolved S-stars and the mysterious G2. The dark matter particles could be 56 keV fermions, which they call "darkinos".
The team also proposes that, above a critical mass, a dark matter clump could gravitationally collapse into a supermassive black hole, like the one in M87.
Is this dark matter model possible? Do the flares from Sgr A* observed in recent years support or contradict this model? Can LHC or other smaller particle colliders or detectors on Earth help to confirm or rule out the particle?


texts: Is the age of the universe relative to an observer's location in that universe?

texts:
According to Wiki the age of the universe is 13 billion years old, and I was taught that background radiation made the universe uniform in all directions.

Doesn't this define a sphere of space in the universe with Earth at the center? This sphere would have a radius of 13 billion light years.

We (on Earth) could not see anything outside this sphere. As light did not exist more than 13 billion years ago. So for us this defines some kind of edge for the universe.

But what happens when an alien in another galaxy 5 billion light years away from Earth also looks towards the stars. Don't they see a uniform background radiation and measure the same age of the universe?

That alien would also have a sphere of 13 billion years.

So we have two spheres, one for Earth and one for our alien friend.

These two spheres would overlap each other by 5 billion years. Which means, if we look in the opposite direction from that alien's galaxy. We can see an additional 5 billion years further than they can.

If we measure the total width of the two overlapping spheres we get a distance of 18 billion light years.

I now grasp my hair in confusion. Is this possible that two visible spheres could overlap to create a distance longer than the oldest ray of light? And from there I fall into more questions. Can two people measure the same universe age from different spots in the universe.

That's why I think my idea is is wrong, but how could it be wrong and why?


texts: What's the useful magnification for a scope with 70mm objective lens?

texts:
Some of the spotting scopes which are e.g. 70mm claim to magnify terrestrial objects at 75x. But yet they cannot produce sharp and clear images at even 30x-35x.

While 90mm scopes can do a better job than them but still they stop  producing clear view at higher magnification.

So, I want to know is there a formula from which I can estimate (if not accurately tell) their useful magnification at which they are able to focus objects?

I came across this:

Magnification = Focal length / Eye piece focal length

But not sure if that works for what I am asking.


texts: Supermassive black holes at the center of galaxies

texts:
The fact is, many, if not most large galaxies have a supermassive black hole at their center. My question is why? Is it because when these galaxies were first formed supermassive black holes were created out of super-dense clouds of matter, which then attracted the rest of the galaxy? Or is the black hole attracted to the center of the galaxy in such a way that due to its large mass and ability to perturb everything around it, the interactions with the galaxy tend to move the center to where the black hole is located?

It seems interesting that Sagittarius A* is at the precise center of the galaxy, when it has so much less mass than the rest of the galaxy put together. The sun being at the center of the solar system makes a lot of sense - it accounts for 99.8% of the mass of the solar system. But the mass of Sagittarius A* is on the order of one millionth the mass of the galaxy.

So, to phrase it like a chicken and egg problem – what came first, the super-massive black holes, or the galaxies surrounding them?


texts: What is the difference between our time and space time?

texts:
I am trying to understand the phenomenon of space-time. But, everything on internet seems to be too complicated for me since I do not have a background in physics. Can anyone give me simple explanations for the following:

What is the difference between time and space-time?
How does gravity affect the passage of time?
What is the speed of light and how does it relate to time?
How do scientists deal with timescales on the order of billions of years if time is not constant for all observers in the universe?
How is time, or for example the age of the universe, actually measured experimentally?


texts: What is the long term fate of the gas giants?

texts:
If I'm not mistaken, it is believed that the reason for such turbulent weather on the 4 outer gas giant planets is that the internal pressure is so great that it is generating heat, which is causing convection, which causes extreme weather.

Will these planets forever generate heat, or at some point will they freeze up?

What will Jupiter look like a trillion years from now?


texts:
We might expect to not find life around halo stars (“Population II” stars) because:

a)
they are out of the plane of the galaxy and thus exposed to more cosmic rays (energetic
nuclei)

b)
these stars are all too hot and thus will expose any planets to too much ultraviolet radiation

c)
these stars lack the heavy elements (“metals”) out of which life can form

d)
these stars have existed too short a time for life to have evolved

e)
the stars all pass too close to the central supermassive black hole in the Milky Way

I am stumped on this question, any ideas?


texts:
For a lot of my uninformed life, I have doubted the existence of gravitons or even that gravity is an actual "force" (like electromagnetism). This is because my vision of general relativity was that mass curves space such that objects are still traveling in a "straight line" when being acted upon by "gravity", so that no "force" is needed. I know now that this is a naive view, but I'm not 100% sure why. I was thinking the other day that just the fact that gravity follows an inverse square law implies that it is a force carried by particles (falling off in flux intensity due to the geometry of 3D space).

My question would be: Does the fact that gravity follows an inverse square law naturally fall out of general relativity equations or is it an assumption used when developing the equations?

And, just now, I had the thought that other forces may curve space also (just in higher dimensions).


texts: In System KIC 9246715, How Far Does One Star Orbit the Other?

texts:
I was going to ask if it's physically possible for one red giant to orbit another, but fortunately, I have just found such a thing--a red giant binary system known only as KIC 9246715.  In this article, all I could find of the distance between the stars is that one orbits the other in an orbit of 171 days.
The only problem with that statement is that it doesn't tell how far one star orbits the other in miles.  In our solar system, a 171-day orbit would be midway between Mercury (orbital revolution:  88 days) and Venus (orbital revolution:  225 days).  Obviously, this couldn't be the case for two stars that are each bigger than our own sun.  But even binaries confer to Kepler's Third Law of Motion, that the farther one body is from its parent, the slower it'll complete one revolution.
So in this real system, have there been any estimations of how far one star orbits the others measured in miles or kilometers?


texts: Our Solar System’s interstellar movement in relation to the Alpha Centauri System

texts: Our solar system is moving at approx. 52,000 mph, so where is the Alpha Centauri system, which is assumed to be also moving through interstellar space at a similar velocity since it is part of our Galaxy. Is the A. Centauri system moving in front of us at 12 o’clock, parallel to us at 3 o’clock, or some other configuration?


texts: Unbelievably high masses/ densities in NASA website data

texts: In May, I did my own amateur analysis of Kepler data that was supplied here. This was a table of all confirmed "Kepler planets" to date (April 1, 2015). This table had some unbelievably high masses/densities listed in the first column after the planet designation (ie: planets of Kepler- 23, -24, -25, -27, -28, -32, -39, -48, -52, -54, -57, -58, -59, -60 and others). I didn't take these seriously, but would like to know if there was a "units" error or something I am missing. To give you an idea, Kepler-23b is given a radius of $1.9 R_{\text{Earth}}$ and a mass of $254.3 M_{\text{Earth}}$ - and that's nothing compared to some others. I checked just now and the data is the same.


texts: How do you find the altitude of the Sun if you are on the Moon?

texts:
The question is:

  If you stand on the equator of the Moon, at midday your position occurs when the Moon is located at the line of the node. What is the altitude of the sun at that moment?

Information: This is when the Moon is located at the line of node (line of node is a line that occurs from crossing between the ecliptic plane and the Moon's orbital plane, which makes an angle of approximate 5 degrees). The equatorial plane of the Sun makes an angle of 1 degrees, 32 arc minutes and 40 arc seconds.

The solution is 88 degrees, 27 arc minutes and 20 arc seconds through 90 degrees.

But I don't understand why the answer isn't 1 degree, 32 arc minutes and 20 arc seconds, because the ecliptic plane where the sun is located is above the horizon or celestial equator. equal to its altitude.


texts: How did we measure the mass of the universe?

texts:
In an wiki article, observable universe, it was mentioned that,

Mass (ordinary matter) 1.5×10^53 kg

I wonder how did we measure the mass of our observable universe?


texts: Is there mass i.e a physical bottom to a black hole?

texts:
All matter has a gravitational pull, a ball, a rock, a planet and a star - the bigger the matter the more it affects the space fabric around it thus having a greater gravitational pull.

I see black holes in the same kind of concept - matter that warps the space fabric/time around it - however as we can physically see a ball, a rock, a planet and a star I fail to visualize what would be the matter that continues a black hole to operate because we cannot see any physical element to it.

EXAMPLE

A black hole is cause when a star implodes on itself and crushes its self to an extent that gravity has such an effect on space time that we know of nothing that produce such a force.

What I would like to know is:

1: Is there a known "bottom" of a black hole? If so, what happens to all of the matter it swallows? surely it would be impossible to crush something so small that it physically no longer exists - where would it go then?

2: How small does the core of a collapsed star crush itself into? i.e the size of an atom or smaller?


texts: Are white dwarfs the ones always absorbing mass from a companion?

texts:
It seems to me every time I see stellar remnant binary systems where there is a white dwarf and another star or something, it appears that the white dwarf always pulls mass from the star.

What if the white dwarf is with a neutron star? Will the white dwarf draw mass from the neutron star? What if the white dwarf was orbiting a supermassive star(not giant stage yet)?


texts: Why do things float in space, though the gravity of our star is always present?

texts:
In the solar system, things float around. We are so certain that the gravity of our sun exists. Still, why does that gravity not influence satellites and other objects there?

Our solar system is also orbiting the center of our galaxy, and the gravity of our black hole, but still many objects float around. I wonder why.


texts:
I've recently watched one conversation (I won't post a link since the conversation is not in english) involving a physicist (at least he claims so), who spoke about probabilistic estimates of the Solar System formation and the birth of life. Which are:

There are approx. 2000 planetary systems discovered, and all of them, except the Solar System, have angular momentum mostly in a star, which agrees to Kant-Laplace theory (probably it's the Nebular hypothesis). But the Solar System is the only one amongst them with angular momentum mostly in planets. This was sort of explained by the Schmidt's hypothesis (it's about the Accretion as I understand), that the Sun was flying through some debris and "picked" some. But the probability of such an event is very low, given the speeds of participating celestial bodies, read quantity of moving stars with requisite speed (why those stars have that speed is also unknown), and also there should have been supernova explosion at the same time. So the Solar System formation explanation is based on the chain of unlikely events.
Earth is the only planet in the Solar System that has continental crust and oceanic crust (I guess it is important for the water cycle and life formation on Earth in general). The only explanation of that fact also relies on the chain of low probable events:

once Earth had an Lagrangian satellite (near Earth), comparable to Mars in size, which is itself unlikely, since Lagrangian satellites are typically much smaller;
each of them were each other's Lagrangian satellites, which is also unlikely, nothing like that was discovered up to date;
then some unknown power brought out that satellite from Lagrangian point. Such power could have resulted from an event of some wandering star or another massive object moving through the plane of the Solar System, which is nearly impossible since even at the event of galaxies collision, the probability of collision of stars is vanishingly small;
as a result, that satellite "rubbed" Earth in an event of collision, tore off some amount of crust, that was how place for oceans was formed, also the Moon was formed from the resulting debris.

Without the chain of events above the birth of life as ours is impossible, and since the aforementioned planetary systems have some consequences of formation according to Kant-Laplace theory, there is no intelligent life possible there.

Are those statements true?


texts: can moon rise before sun and set after it in the same day

texts:
I found it mentioned in ancient Arabic astronomic book Kitāb al-Amwāl by Ibn Qutaybah (889 AD)

Cresent can't be seen in the morning in the east in front of the sun and in the evening i the west behind the sun in the same day

ولا يمكن  أن يرى الهلال بالغداة فى المشرق بين يدى الشمس، وبالعشىّ فى المغرب خلف الشمس فى يوم واحد.

Is this information valid for all locations and dates
Sun and Moon have very near orbits and both daily apparent motion (east-west) but moon has apparent retarding motion (west-east) could this retarding motion make the sun catch the moon in the first day of lunar month?
could this happen in normal (non polar) latitudes?


texts: How was precision astrometry done before digital imaging?

texts: Did they just put a ruler on the photographic plate? Quite some precision must have been required to discover Neptune from its influence on Uranus for example.


texts: Tycho Brahe Danish Astronomer Religious?

texts: Does anyone know if Tycho Brahe was religious?


texts: How does one calculate inclination from a graph of radial velocity?

texts:
If a binary star system has a non-zero inclination, how would that show up in a graph of radial velocity?

How could I then extract the inclination from the graph?


texts: Multi-messenger astronomy: what is the potential of simultaneous detection of gravitational waves and neutrinos from a supernova?

texts:
Thanks to the efforts of the aLIGO team, gravitational wave astronomy is a reality. At the same time, neutrino detectors like Hyperkamiokande are becoming much more sensitive.

My question is: what are the prospects for the pseudo-simultaneous detection of gravitational waves and neutrinos from the same supernovae? What sort of stuff could we learn from such an event, both about supernovae and neutrinos? In particular, what are the prospects for estimating the neutrino mass?


texts: Venus has a greenhouse affect because its atmosphere is 96 percent carbon dioxide. However, although mercury is closer to the sun it does not have the greenhouse effect because it has no atmosphere and only an exosphere. I was wondering why doesn't mars have a greenhouse effect like Venus? They both have around 95 percent carbon dioxide in their atmosphere and consist of an atmosphere. In addition, I was also wondering about what are the requirements to have a greenhouse effect on a planet?


texts: Does a kilonova leave a high mass remnant?

texts:
My understanding of Type Ia supernovae is that they are expected, in most cases, to destroy the white dwarf(s) that went in to them, leaving behind no high density remnants (i.e. no white dwarf, neutron star, or black hole). Black hole/black hole collisions are expected to leave behind a black hole, of course, with less mass than the sum of the masses of the black holes that went in to the collision. Do we expect a kilonova to leave behind nothing but gas and radiation or some kind of stellar remnant? If it leaves behind a remnant, what class to we expect it to be (white dwarf, neutron star, or black hole) and what mass? The addition of mass seems redundant, but the dividing lines between the masses of these objects are based on upper limits on the mass of the less dense class (Chandrasekhar limit for white dwarfs, Tolman–Oppenheimer–Volkoff limit for neutron stars), and don't actually apply as lower limits for the mass of the high density class. For example, neutron stars are sometimes referred to as giant nuclei, which would put the lower limit of their mass at $1$ or $2$ atomic mass units, depending on whether the presence of a neutron and stability to radioactive decay are requirements. Yes, those times people are being poetic due to the difference in stabilization mechanisms (gravity vs nuclear forces), but the point remains that it may be possible for a neutron star to be theoretically stable at less than $1.4M_\odot$. The only lower limit to black hole mass I know of would be the lifetime limit from black hole evaporation.

Put another way, do we think the density at any point in either of the inspiraling neutron stars gets high enough to form an event horizon? If that happens, this seems like a plausible way for producing black holes that are smaller than those produce by core collapse supernovae, kind of like how critical mass for atom bombs can be achieved by either bringing together enough fissile material or compressing the available material enough (e.g. the fat man detonation mechanism vs the little boy one).


texts: Would a human body float in the dense atmosphere of Venus?

texts:
To survive high in the atmosphere of Venus, all you would nead to wear is suit that protects you from the sulfuric acid vapors in the air and a supply of breathable air. Assuming (for simplicity) that this gear has the same density as the human body (the air supply would probably much heavier), could you float on top of denser gas layers below you?

I believe the density of the gas would only have to be about as high as the density of water on the surface of the sea. And would that mean that when you pour out a bottle of water, the water would float in the air (given the temperature/pressure ratio is below the boiling point)?


texts: Array of telescopes with single orbiting collector?

texts:
Might such a thing give better images than we could get from other virtual telescopes with multiple collectors? Or is having multiple collectors not a limiting factor? Could a single collector compensate for atmospheric distortion in a way multiple collectors cannot?

We sometimes use arrays of radio telescopes spread over a wide distance to increase the effective aperture. I think that array is called a virtual telescope, but I might be wrong. A normal radio telescope has a collector in the same way that a satellite dish has a collector. The reflector of a radio telescope is parabolic so as to focus parallel waves to a single point. That works because dish and collector are in a fixed geometry. The reflectors of my imagined telescope would need to turn so that they reflected waves from whichever part of the sky was being observed onto the collector. That turning would change the required curvature for perfect parabolic-ness.

The telescope I'm imaging would have a (perhaps geostationary) satellite as the collector and an array of flat-ish reflectors spread over hundreds of kilometers working together to collect waves from the same part of the sky. Being non-parabolic, the reflectors wouldn't be very efficient, but they could compensate by being numerous.

The collector wouldn't have to be geostationary, because the reflectors already need to compensate for the earth turning; adding the satellite's movement into the equation wouldn't be terribly complicated.


texts: Does the Sun have the same composition and density everywhere within it? How does it vary in other stars?


texts: Saw a bright orange object in the sky. What was it?

texts: Some friends and I were backpacking in the mountains, and at night we saw a very bright orange object in the sky. It looked brighter than Venus. After a couple minutes we realized it was moving horizontally very slowly. Its movement wasn't noticeable unless looking at it next to a reference point like a tree. This was way brighter and slower than any satellite I've seen. Anyone know what this was?


texts: Is there a comfortable part of Venus's upper atmosphere?

texts:
I know the surface of Venus is very hot, but that it gets cooler the higher you go in the atmosphere.  I have heard that there is a section in the atmosphere of Venus that might be somewhat habitable even, that where it is about 1 atmosphere, the temperature is slightly over 100 F.  Not really comfortable, but possibly livable temperature wise (likely with some cooling mechanism).  However, I recently heard that near the poles, the atmosphere is extremely cold: http://www.iflscience.com/space/death-plunge-venus-spacecraft-reveals-hottest-planet-not-so-hot/

Does this mean that at the height where the atmospheric pressure is 1 atmosphere, and somewhere between the pole and the equator would lie a comfortable zone of 72 F?  "Extra credit" for suggesting / providing reasoning for a likely latitude where this condition would exist (if it does).  If this does exist, it might make an ideal place for a blimp like sky colony.  I am considering adding this to my Planetary Settlers video game.


texts: What increases or decreases the mass and density of a fixed radius star?

texts:
Assuming a relatively even proportion of mass and radius, a 0.25 solar masses and radii star would have a density of 22.5003 g/cm³, or about 16 times our Sun's density.
Keeping the radius of 0.25 solar radii, what influences would cause different 0.25 solar radii stars in the main sequence to have higher or lower masses/densities than the "normal" 0.25 solar masses?


texts: Why is the outline of a crescent moon brighter?

texts:
Some quick googling has given me the answer to whether seeing part of the moon that receives no sunlight is an optical illusion or not: it isn't. Earthshine lights the moon enough for the human eye to notice.

However, it seems clear to me that the outline of the unlit part of the crescent moon is brighter than the surface. I was riding my bike home yesterday on a clear winter night, and became confused on why that could be. What mechanism is responsible for making the edge brighter than the dark part of the moon? Or is it my brain trying to make a circle?

Here's an example of what I mean.


texts: After what time interval do the closest approaches of Mercury to the Earth repeat?

texts:
The sidereal period of Mercury's revolution is 88 days and the synodic period — 116d.
my solution, but in the question featured "the greatest rapprochement." And this is no longer so easy. Because the orbit of Mercury is noticeably elongated and ... in turn, it experiences rotation around the Sun. The effect is called "precession of the perihelion of the orbit of Mercury". We must take it into account. The closest approach of Mercury to Earth will be when it is at this point, and it, in turn, is on the Sun-Earth line.

The time of Mercury's revolution around the Sun is 88 days,
Earth around the Sun -365 days,
Let's say we are on Mercury and today there is an Earth opposition,
the next one will be when Mercury returns to its original place
(having made a circle in its orbit) it is 88 days, but these 88 days
the Earth does not stand still, it has shifted in its orbit by 88/365
by about a quarter => Mercury will have to "catch up" with the Earth
for 22 days, but during these 22 days the Earth will still shift in
orbit by 22/365 approximately one sixteenth => another +5 days. Total
88 days + 22 days + 5 days = 115 days
Answer: oppositions of the Earth for an observer with Mercury are
repeated with a frequency of 115 days

I found the answer 7.26 years but how do get it?


texts: need data-point: actual counts per second with APD (avalanche photodiode) detector

texts:
I'm designing a sensor system to perform specialized [astronomy and space-sciences] experiments, and need a "reality check" to support or adjust my theoretical calculations.

What I need is the "counts per second" produced by any modern APD (avalanche photo-diode) sensor through a telescope of any specific "aperture" of a star of any "visual magnitude".  I also need the number of "counts per second" of "nothing" (the "dark count") to subtract that from the "counts per second" when illuminated by the star (to determine the "counts per second" generated by the star alone).

This "reality check" will help me assure various "inefficiency allowances" I made are realistic.  Examples:

#1: overall detector QE over relevant visual [and near IR] wavelengths.
#2: loss of light in atmosphere before entering telescope.
#3: loss of light in telescope optics.
#4: loss of light in fiber (if any).
#5: anything/everything else.

As implied, I am only interested in the APD operating in "photon counting mode" (not analog).

I've read about 5 dozen articles that discuss APDs for astronomy, but none give a straightforward value.  The closest I found was a vague statement that the limited magnitude was 22nd magnitude on a 6-meter telescope based upon observations of the crab nebula pulsar.  But this is not specific and the object is highly variable (on a short time frame).  They did not say, for example, whether they consider their "limiting magnitude" is where the count rate rises from 200 per second (dark count) to 220 per second (measurement), or 200cps to 400cps, or over what time period, or any other indication of their definition.

All I need is ONE clear statement of cps for any aperture and visual magnitude star.  You'd think I could find that in dozens if not hundreds of articles, but... no.  Probably a clear statement like I need exists in some article somewhere, but I haven't seen one.  Have you?  Or better yet, have you made such an observation yourself?

The following detail is not very important (but just to be complete), my primary applications perform fairly high time-resolution measures on fairly bright stars.  In other words, the experiments generally need to measure in the range of "counts per microsecond" to "counts per millisecond".  Typically APDs max out at around 15 to 50 million counts per second, and most of my experiments will be working at 10K to 10M counts per second to observe the short time-period phenomenon I need to measure.


texts: Did Cassini return a photo of Saturn's rings shown from closer to Saturn?

texts:
A quote from a book, Perelandra by C.S. Lewis: "no eye looked up from beneath on the Ring of Lurga"; now Lurga is Saturn, and no human eye has been to Saturn, let alone at a lower altitude than the rings, and quite probably would not survive the radiation exposure.
But we have an electronic eye; that is the camera, and on board Cassini were two cameras  and these have returned photos of marvelous views of Saturn. I tried a search to find if any photos were taken looking at the rings or otherwise showing the rings while Cassini orbited so close on its final orbits, but I found only artwork.
Were any such photos taken?


texts: Can someone explain this diagram showing the spectral type distribution of bright stars

texts:
Can someone explain this diagram?

The text is in Dutch, free translation: "You are given a graph: a histogram of the 10 000 most apparent bright stars. Explain 1, 2 and 3."

The fact that the number of counted stars of types O and M is small due to

O is bright but just very rare and,
M is common but quite dim.

The puzzling bit in this diagram is the sudden dip from a local maximum F (why is there a maximum) to G (why is G a local minimum). The drop from K to M is explained above - that being that M class stars are just very dim (is this all there is to say here?).

It seems that if there wasn't a dip at 3, and the graph would just continue, that the local max at K can be explained as being there just because there is a dip at G. If there wasn't a dip it wouldn't be a maximum and it'd just look like a bell-curve.

So

Why is there a dip at G?
Why does it rise so smooth from O to F? Is it just because B - F are more common?
Why is there a maximum at F?


texts: Pointing direction movement of a fixed camera

texts:
first doubt here:

I have a fixed camera, whose coordinates I know, pointing to the sky. After the first image, I performed a astrometry calculation from nova.astrometry.net and I got, among other things, the Ra and Dec of the center of view. I would like to use the calibration information for other pictures without needing to calibrate again, e. g. copy the calibration info and just update Ra and Dec of the next pictures.

I thought I could do so transforming the Ra and Dec to Az and Alt using the Julian Date of the first event(the calibrated one), and then go back to Ra and Dec using the Julian Date of the second event. But I get wrong results. I tried between different pictures and sometimes the error is bigger some time smaller.

I am using the functions eq2hor and hor2eq of Astrolib (IDL), which takes in account precession and nutation, so I assume than there is a gap in my knowledge and procedure, rather than a programming mistake.

What am I doing wrong?

Thanks in advance


texts: Was NASA's 2007 attempt to see helions during a solar eclipse successful? What instruments were used?

texts:
This answer about Sun-grazing comets led me eventually to Wikipedia's Helion (meteoroid), which has a one-sentence long article:

  A Helion meteoroid is a meteoroid that arrives from the approximate direction of the Sun. They are thought to originate as debris from sun-grazing comets.

It has one reference

  "Exploding Lunar Eclipse", an article about an attempt to observe Helion meteoroids, Science@NASA, August 27, 2007.

but that link is now dead.

Was NASA's 2007 attempt to see meteoroids coming from a solar eclipse successful? What instruments were used?
Has it been attempted again since?


texts: If an object was a distance from a planet away that equaled the distance from the object to a moon of the said planet, would it be drawn into the direction of the barycenter as it is the common center of mass?


texts: Why can't we see a solar eclipse every month?

texts:
In every lunar revolution around Earth, the Moon is at a point between the Earth and the Sun. Then why can't we see a solar eclipse once every lunar month?

Is it because of a tilted orbit of the Moon around the Earth? Is the orbit of the Moon some kind of a sphere?


texts: Why are distant galaxies not visible in the observable Universe?

texts:
Assuming that the Universe is and has been expanding ever since its creation (Big Bang theory), the galaxies that are now at the 'edge' (not visible theoretically) must have been (at some point in time) at a place around where the Earth is at now.

If it is assumed to be true, then the photons emitted by these galaxies from this point onwards should be around for us to see these galaxies (and even the first light emerged from the Big Bang must be visible).

Then why is the observable so small? It should depend upon the direction in which we look. That is, if we look in the direction of the center of the expanding sphere, it should be a small observable universe. If we train our telesopes in 'exactly' opposite direction of the center, it should be 'fully' observable.

Does it work this way? Or are my basics wrong?


texts: Why Earth is like a magnet? how can earth attract any object irrespective of what it is made of? What is the reason behind Celestial Body's gravity? i.e Sun,Jupiter,Moon etc.


texts: How do we know galaxy GN-z11 is as far away as it is?

texts:
The news media recently reported that galaxy GN-z11, formed just 400 million years after the big bang, is much further away than originally thought at perhaps 13.2 billion light years.  They say,

  The key to the discovery was precisely measuring the shift of the
  galaxy's light into longer, redder wavelengths, which correspond to
  how far the photons had traveled before reaching Hubble's eye.
  (Reuters).

So, suppose I'm standing next to, but safely away from, some train tracks.  As the train approaches, sounding its whistle constantly, the pitch will appear to increase until it reaches me, upon which I will briefly hear the whistle's 'rest pitch' (the pitch of the whistle as if I were aboard the train), then it would decrease as the train moves away.  Thus, the lower the pitch, the further away the train is.  I hope that's right.  So, if we know the rest frequency of galaxy GN-z11, its measured frequency indicates how far away it is from earth. Trouble is, how do we know what the rest frequency of light coming from this distant galaxy is?


texts: How do astrophysicists handle the topic 'boundaries'?

texts:
The naive perception, at least mine, is that in astrophysics the objects of investigation evolve in an infinitely large environment. Since the scenery is so much larger than the events unfolding in it, astrophysicists should never have to take into account that the universe may have walls/envelopes/surfaces limiting the going of things. I wondered to which extent this vision is naive.

How do astrophysicists handle the topic of 'boundaries' in space?
This is a very general, open, `soft' question, almost philosophical. Handle is a jolly word to mean think of, take into account, represent, model, simulate, even avoid, and so on and so forth.

Clearly I am neither an astronomer nor an astrophysicist.


texts: I have a Celestron 60LCM refractor telescope. It has a 60mm aperture and a 700mm focal length. It came with two eyepieces. One is 25mm and the other is 9mm. They are marked only with their focal length - there is no brand name or part number printed or stamped on either. I can view Jupiter and Saturn through both eyepieces. However, even though the 9mm should have a higher magnification, the planets actually look smaller when looking through it than they do when viewing them through the 25mm eyepiece. Is this a field of view issue? And if so, what field of view should I look for when I shop for new eyepieces for viewing Saturn and Jupiter?


texts:
I know light, and practically nothing but gravity can escape a black hole.  My question is: can magnetism escape a black hole?

A couple things that convince me it can are:

Jupiter's magnetic field shape compared with the jets that can come from near black holes (I would think this might push material falling toward a black hole into or out of the poles):

Apparently black holes have very strong magnetic fields:

http://www.iflscience.com/space/magnetic-fields-can-be-strong-black-holes-gravity


texts: What does it mean 'black holes is created when center of a very massive star collapses in upon itself'?

texts: I read many times that black holes is created when center of a very massive star collapses in upon itself but what does it mean ? Does weight of center of massive star increase so much that itself can't bear weight or I can say space itself cant bear weight of such center of massive star that it collapse. I tried to read answer on google but want to know in layman language as I am not astro physics researcher or student. Thanks


texts: So where are these measurements of galaxies moving faster than light?

texts:
https://physics.stackexchange.com/questions/400457/what-does-general-relativity-say-about-the-relative-velocities-of-objects-that-a

"we can actually observe galaxies that are moving away from us at >c"

Um, I think I missed the groundbreaking headline that said scientists have directly measured a galaxy moving away from us at speeds faster than light.

So the first answer seems to contradict this other answer

https://physics.stackexchange.com/questions/107748/how-are-galaxies-receding-faster-than-light-visible-to-observers

"If recession velocity at the location of a traveling photon were greater than the speed of light the entire time the photon from a distance galaxy were traveling, we would never observe the photon."

It seems like common sense that at a certain distance, space is expanding faster than light, thus preventing light beyond a certain distance from reaching Earth which means we can't directly measure galaxies traveling faster than light, but we can imply that they do by projecting their velocity beyond the observable universe. But who's right?

If a galaxy used to be within the observable universe, we can measure photons from that galaxy reaching Earth despite that that galaxy may be currently outside the observable radius of the universe. Galaxies can be projected as travelling faster than light due to the expansion of space, but I haven't seen that we can directly measure them as doing such since the redshift of photons is proportional do the amount of space that is expanding between us and a certain distance.


texts: Predicting/simulating magnetospheres of exoplanets?

texts: Based on atmospheric parameters, size, or distance from the star, is there any way to simulate or predict the magnetosphere of an exoplanet? Are there programs (similar to climate models for atmospheres) that can simulate magnetospheres?


texts: Is it possible to have two objects moving by sum speed of light (c) in the opposite direction?

texts:
I am puzzled by this question, because by the laws between the two object the relative gravitation force should be infinite. Anyways we can take two objects where the sum of speeds are equaling c (speed of light).

So the answer should be not, but many theoretical physicists making examples to demonstrate what happens if we closing to the speed of light. But what happens if we reach 90% of the speed of light and an object flies on the opposite direction by 10% of speed of light? By rational thinking there should be an extreme gravitational force between the two items.

All of these are true if I understood correctly the the General Relativity Theory.

Can somebody point out what is the problem with my thinking if there is?


texts: How to reduce magnification and increase FOV of a camera attached to a telescope?

texts:
I am a beginner in astronomy.
I have a Maksutov telescope (90mm/1250mm). Generally I do observation with a 25mm wide angle eyepiece, but sometimes due to light pollution I can't see many stars.
So I modified my webcam and converted to a telescope camera.
I can take detailed pictures of moon craters, but the FOV is very small. Is there any way to increase FOV? Can I set the camera on the eyepiece?
note: I am not expecting high quality DSLR-like images, but expecting more stars due to a longer exposure time.


texts:
Just more of a conceptual question on the mutual inclusivity of the cosmological principle. That is to say, I was wondering if it were possible to have a Universe that were isotropic but NOT homogeneous OR a Universe that were homogeneous but NOT isotropic.

My spidey sense is telling me that it is possible that a Universe could be istropic but not homoegenous.

Surely, by default, if a Universe is homogenous then it follows that it has to be isotropic?


texts:
Recently I looked into the coldest(and hottest, but in another question) natural places. I discover this to be the Boomerang Nebula, with a freezing temperature of about −272.15 °C. Then I looked at the reason.

The outflowing gas is moving outwards at a speed of about 164 km/s and
expanding rapidly as it moves out into space; this gas expansion
results in the nebula's unusually low temperature.

https://en.wikipedia.org/wiki/Boomerang_Nebula

This allows the gas to lose its heat. It works on the same principle
as that of refrigerators where expanding gas allows cooling.

https://factslegend.org/20-interesting-boomerang-nebula-facts/

Now I am sort of confused. How exactly can expanding gas lose heat, and lose it to such extent? And gas should cool to the "universal surface temperature", or the cosmic microwave background temperature if it stays there. However, it cools 2 °C lower. How does this happen? Would there be bose-einstein condensate matter there?


texts: The Sun's position at noon is perpendicular to the local x-y plane on the equator during equinox. So the declination of Sun on that day is 0. Today the declination of Sun is close to 23, can this be taken as a virtual equator and hence the tropic of Cancer is in 46.5N (by adding 23.5 to it) and tropic of Capricorn is in 0.5S? Or my assumption is wrong?


texts: A good telescope for the viewing of Nebulae, Stars and Planets

texts: So, I am a 15 year old interested in astronomy. My father used to have a reflector telescope, but was never so serious about it that he could recommend me a good telescope. I am looking to buy a telescope up to £400 (possibly £500) and would like one that is mainly great for looking at nebulae. Stars and Planets would be great also, but are not necessary. Light Pollution isn't too bad. I have been recommended a Newtonian Reflector, but am not too sure if this would be suitable for my conditions. Also, does anyone recommend a good, British website where I could buy such a scope from? Finally, is there much point in me buying a computerized scope, will this allow me to align the scope better, or is it just going to be a waste of money.


texts:
Where can I get a more detailed breakdown of Spectral Classes.  Wikipedia only has it at Letter level rather than sublevel.

For example, https://en.wikipedia.org/wiki/Stellar_classification has B ranging from 10,000 - 30,000K.  B0 is different to a B9, what would be the temperature range for a B0 or a B9 but not limiting to B but for all letters?

I'm not just looking at Temperature, I'm after Mass, Radius, Luminosity, Hydrogen Lines etc.

The closest that I've come is https://sites.uni.edu/morgans/astro/course/Notes/section2/spectraltemps.html  but it doesn't mention types II or IV and doesn't have Radius or Mass.


texts: Recently on the discovery channel I heard that a black hole burps matter out of it when it consumes more than it could process at a time. Given that even light cannot escape from a black hole, how does this "burp" matter escape?


texts: Calibrating raw photometric measurements

texts:
So I have a photometry data sourced from Solar Mass Ejection Imager's (SMEI) server as an ‘add-on data’ for my point source of interest (a variable star) with a 104 minute cadence. However, it’s not really in the UBV photometry system, in V-band specifically, but rather scales with their SMEI photometry system while processed through a centroid fitting.
Is there a possible way to initially calibrate the photometry data like mapping the point source's nearby bright stars perhaps? And if so, how should I start of?
Attached photo is the 'raw' light curve of the star from the gathered SMEI data.

Please bear with me for I'm just a novice especially with regards to photometry. Guide(s) and response(s) are always welcome and I look forward to.
Clear skies.


texts:
Today, I thought to make a list of Astronomical objects, A to Z.

It goes as follows :

A - Asteroid
B - Black Hole
C - Comet
D - Dwarf Planet
E - Elliptical Galaxy
F - Falling Star
G - Globular Cluster
H - Hypergiants (Stars)
I - Interstellar Medium
J - Jets (Astrophysical)
K -  $\color{red} ?$
L - Lenticular Galaxy
M - Meteor
N - Nebula
O - Open Cluster
P - Pulsar
Q - Quasar
R - Rogue Planets
S - Satellites
T -   $\color{red} ?$
U - Universe
V - Variable Star
W - White Dwarf
X - X-ray Star
Y - Yellow dwarf star
Z -    $\color{red} ?$

But I couldn't think of something well-known from the letters K, T and Z.

I tried to avoid using name of something specific object for ex. K - Kuiper Belt.

Can you please help me in completing this list?

Thanks!


texts: Why don't we detect planets around OB stars and no terrestrial planets around A or early F stars?

texts: Looking at an exoplanet database, I noticed that there are very few planets detected around main-sequence OBA stars, and most of them are gas giants/brown dwarfs. Why can't we detect low-mass planets around these stars?


texts: Is Planet Nine shepherding the Kuiper Belt?

texts:
Is it possible Planet Nine is shepherding the Kuiper Belt similar to the way Jupiter shepherds the asteroid belt?

I understand that very little is actually known about Planet Nine at this point, but has it been postulated that it has control over the Kuiper Belt? I don't exactly intend to read any theoretical papers on the existence of Planet Nine because I am sure it will be way over my head, but I understand that it is from the position of several Trans-Neptunian objects that its existence has any merit at all.  So, I am wondering if part of that theory includes the Kuiper belt as a whole?


texts: Are our 'observations' of or near a black hole affected by gravity induced time dilation?

texts:
This answer made me wonder if matter/energy/events near a black hole are viewed (by us) in a very different time frame, how might that affect our measurements?

The people on the smaller planet would observe the people on the extremely large planet
moving in "slow motion". (note: due to gravitational time dilation)

On a related note, does light being 'bent' around a black hole experience time dilation or red/blue shift from our viewing perspective? (as possibly suggested by this answer).


texts: Can we visually perceive exoplanet transits with amateur telescopes?

texts:
Is it possible for an amateur astronomer to visually perceive an exoplanet transit by the change of brightness of the star via observations spaced over time, or is the variation of the brightness too insignificant to be seen?
I have a 70mm scope, but no astrophotography equipment or software to compare data. My question is about really perceiving the brightness variation with our own eyes over time as we see it on variable stars.


texts: Is there a closest free-return trajectory to a black hole?

texts:
The scenario I am imagining is a test particle falling from infinity with an initial off-center velocity. In Newtonian mechanics the path would be a hyperbola. But I assume that, near a black hole, that is no longer a good approximation.
I have read about the innermost stable circular orbit, and it seems like there might be an innermost free-return distance, where any object that falls within that distance will need to use rockets to get back out. I may have even come across this idea, but I am not finding it with a Google search.
I have an argument that there must be such a distance. Imagine that you could fly past a supermassive black hole just half a meter above the event horizon and continue on to deep space without using rockets. Tidal forces aren't so great at the event horizon for a SMBH, so there should be no reason you can't extend your arm just over half a meter down and not pull back a bloody stump. But you can't retrieve things from below the event horizon. This paradox is resolved if, in fact, the only way to escape from that close to the EH is to exert massive force using rockets (or some other method). Even though you are not yet inside the EH, your free fall path will take all of you there, and the force to prevent that would pulp you anyway. I think it is related enough to the main question to ask: Is this analysis correct?


texts: Clear boundary between core and shell of a gas star

texts: Does a gas star have a clear boundary between its core and shell? If it does, then what is keeping a helium core distinct from its hydrogen shell, for a main-sequence star that transforming to a giant star?


texts:
I've read that when a body reaches about 13 times the mass of Jupiter that deuterium fusion starts and the body becomes a brown dwarf instead of a planet.

I have a multiple question.

Are there any other factors besides mass and possibly metal abundance (composition) that would determine this "turn on" point ?
If the "brown dwarf" was detected indirectly and no spectroscopic information was available, could we assume that metalicity would be the same as the larger star ?
Would a higher metallicity body "turn on" at a lower mass? (Would a difference between [Fe/H] = -0.19 and [Fe/H] = +0.07 make much of a difference in "turn on mass"?)

*I did miss the related question when I posted this one, but I do have a question here that wasn't answered. It was about how stellar metallicity effects the onset of fusion. I think it may be as simple as whether or not adding "metals" makes the core hotter or colder for a given mass. I also wanted to know if I could assume that both "stars" in a binary system should have the same metallicity.

*Now that I've read Rob's excellent (as usual) answer, I would be curious as to how long deuterium fusion lasts in brown dwarfs from 13 to 80 Jupiter masses and what happens (in models) when it stops.


texts: What is the highest angle the moon ever makes above the horizon at the North Pole?

texts:
The question is in the title. I'd like to find out the maximum angle the moon ever makes above the horizon at the North Pole. By "the horizon at the North Pole," I mean the tangent plane to the earth at that point.

Note that I'm not asking for the highest angle that the moon ever appears to make above the horizon. Appearances are distorted by refraction, etc., although I'm not sure by how many degrees. Though I'd be interested in the exact distortion: is there a data repository where I can look up the highest observed angle?

I would make the following prediction. The moon's orbital plane is inclined to the earth's equatorial plane at about $23.5+5\approx 28.5$ degrees. Let $R_e$ denote the radius of the earth and $R$ denote the radius of the moon's orbit around the moon. A little geometry for the highest point suggests

$$\theta_{\text{max}}=\tan^{-1}\left(\frac{R\sin(28.5^{\circ})-R_e}{R\cos(28.5^{\circ})}\right)$$

But because $R>>R_e$ (about 385,000 compared to 6300 km), $\theta_{\text{max}}$ should be just a little less than $28.5^{\circ}$.

Is this right?


texts: What will the mass of the new galaxy be?

texts: When the Milky Way galaxy collides with the Andromeda galaxy, what will the mass and volume of the new galaxy be?


texts: How do we know a star's age based on its spectrum?

texts:
A star's nearby environment may give clue to its age.

Different stellar type have different spectral features. If we just have its spectrum, how do we know its age?

For example, a star may be born with a M3-type(I know it is more difficult for M-type compared with OBAFGK) and stay here for a long time.
How do we know it was born with a M3-type?
How do we know its age?

It is better to talk about different stellar types and give reference papers.


texts: Distinction between metric expansion and objects just moving apart from each other?

texts:
I see explanations likes this:

Galaxies and other sources, then, are not strictly expanding away from each other but rather are attached to the fixed grid on the expanding fabric of spacetime. Thus, the galaxies give us the impression of moving away from each other. (Scientific American)
[...] space is expanding [...] the most correct way to think about it would be that the grid points are getting farther apart. (Physics.se)

How have we determined that it is some underlying grid or space itself that is expanding rather than the objects just moving apart on a static grid of spacetime?


texts: What is the completeness of an observation and how do I calculate it?

texts:
Many publications write about "completeness". I have a vague idea that it has to do with the ratio of how many sources from an observation are detected and how many sources are actually in that field. However, can someone please explain to me:
a) What is the definition of completeness?
b) How do you calculate the completeness of an observation?
As an example: Smolčić et al. 2008 are writing about completeness. Please have a look at A New Method to Separate Star-forming from AGN Galaxies at Intermediate Redshift: The Submillijansky Radio Population in the VLA-COSMOS Survey


texts:
What is the correlation between stellar abundance by mass and by number?
Let’s take the Sun and Helium for example. This paper mentions an abundance by mass for helium as Y=0.275 and by number as A=10.99, which I believe is derived by assuming N(He)/N(H)=8.5%.

How do you get this 8.5%? Is there a standard value for N(H)?
Why are abundances  given both by mass and by number? Is there a historic reason?
Is there a relation between the two, that is can you estimate one “type” by knowing the value of the other?


texts: Why isn't the day backwards between leap years?

texts: So every four years, we add an extra day. A leap year. This is because a year is actually $365_{1/4}$ days long. But what happens between leap years? After two years, it would get dark at 8:00 AM instead of 8:00 PM. This is obviously not the case. What's going on here?


texts: 1:1 resonance of two planets around a star in close proximity - approximate perturbative mathematical treatments?

texts:
This question is primarily about the mathematics of orbital mechanics.

The circular restricted 3 body problem (in 3 dimensions) assumes two massive bodies in circular orbits around their center of mass, and a third with zero mass. It is usually solved in a rotating frame and in reduced units with the separation equal to unity and the period of rotation of the frame of 2π. Five Lagrange points can be found from the pseudo-potential surface in 2 dimensions (as well as some periodic horizontal Lyapunov orbits), and things like halo, Lissajous, vertical  Lyapunov and other periodic orbits can be found when solving in 3 dimensions.

My question is about a relaxation of some of conditions of the problem, allowing for the third body to have a non-zero mass, at least so that the 2nd and 3rd body have some non-trivial center of mass relationship themselves.

I'm asking about orbits that would be periodic (or nearly-so). Mathematical stability is not necessary, small displacements can lead to exponential growth of errors, as long as there is at least one mathematically defined periodic orbit.

For visualization purposes, imagine moving Earth's Moon to somewhere in the region associated with the original Sun-Earth L1 point, but that's just to get the conversation going.

In this case, one could think of the "Earth" and "Moon" as both being in heliocentric orbits, but so close that they are in a 1:1 resonance. The "Moon" would orbit the Sun with an average velocity of about 1% slower than that of the Earth so that their heliocentric orbit period were the same.

Has this kind of configuration been explored mathematically? I don't care if it is 3D or only 2D, nor if all six 2-body interactions are considered explicitly or if there are some simplifications.

Basically, anything that considers some effect of the "Moon's" gravity on the "Earth" so that it is no longer treated as massless.

There is certainly a body of work on non-unity orbital resonances; 2:1, 3:2 etc. but has 1:1 ever been treated in some way?

Question: Considering a 1:1 resonance of two planets around a star in close proximity (e.g. large asteroid or small moon at Sun-Earth L1) - have there been any approximate and/or perturbative mathematical treatments?

note: original motivation is this answer.


texts: Long term effect on rocks of high pressure and temperature?

texts:
The mean surface temperature of Venus is about 460 C and the pressure is about 92 atmospheres. Assuming this situation (or similar) has prevailed for millions of years what changes (if any) would there have been in the structure and chemistry of the rocks on the surface of Venus?

Wikipedia(Venus) http://en.wikipedia.org/wiki/Venus


texts: Good source for the relationship between absolute magnitude, diameter, and albedo?

texts:
In this answer I have rearranged an equation from somewhere, and now I can not relocate the source. My re-aranged form is:

$$M_{Abs} = 5 \left(\log_{10}(1329) -\frac{1}{2}\log_{10}(\text{albedo}) -\log_{10}(D_{km})\right)$$

This is the relationship between the absolute magnitude of an object and its diameter and albedo, assuming it is spherical.

Question: What is a good source for any form of this equation to which I can link in my answer?


texts: Why are orbits elliptical instead of circular?

texts: Why do planets rotate around a star in a specific elliptical orbit with the star at one of it's foci? Why isn't the orbit a circle?


texts: Stars moving in line and some moves around

texts: I am from South Africa this morning around 05:00 I saw 7 to 9 stars moving in line and some moves around. What cause that to happen. Because I have never saw  something like that before.


texts:
I have 100cm F.L objective and 10cm F.L eyepiece 10cm in diameter and double convex both.
what are the planetary objects I can see with telescope made with these lenses.

I also have a double convex and double concave lens of 20cm F.L both having 5cm diameter. will any of the above said lens help the telescope I'm going to make if included somewhere in the design?


texts: How uniformly is cometary debris distributed along the entire orbit of the comet?

texts:
Meteor showers occur when Earth passes through debris shed by a comet (if I get that right). Does this mean that the entire orbit of the comet is strewn with debris? I suppose that the debris follows the same orbit as the comet core it was released from as it passed close enough to the Sun for ices of different kinds to sublimate. Is the debris that cause meteor showers somewhat evenly distributed along a huge ellipse crossing the orbits of Earth and (often) Jupiter?

Are such cometary debris streams, and not only those crossing Earth's orbit, somehow cataloged and characterized?


texts: Dataset for machine learning MK stellar classification

texts:
I would like to create program for automatic Morgan-Keenan stellar classification using machine learning. For that, I need dataset of stars with known absolute magnitude, temperature and luminosity type (0, Ia, Ib, ..., VII). I found some datasets, (e. g. kaggle.com), however they contain only few hundreds stars and not all star types are represented.
Is there any large enough dataset (at least 1000 stars) in which all types of stars are represented, from hypergiants to dwarfs, and with all the information mentioned?


texts: What equipment and techniques were used to study Betelgeuse's diameter in 1920?

texts:
Comments below What is the maximum distance measurable with parallax? discuss challenges associated with parallax measurements of Betelgeuse and link to Wikipedia’s Betelgeuse; Distance measurements which contains the intriguing sentence

  In 1920, when the first interferometric studies were performed on the star's diameter, the assumed parallax was 0.0180″.

How exactly were the first interferometric studies performed on Betelgeuse's diameter? What actual equipment and technique was used to do this in 1920?

Was this two optical telescopes and a long baseline?
Are they still around today?
Was this the first attempt to measure a star's diameter directly?
Was this attempt at least partially successful?

Addressing item #4 further, the paragraph goes on to say

  This equated to a distance of 56 pc or roughly 180 ly, producing not only an inaccurate radius for the star but every other stellar characteristic. Since then, there has been ongoing work to measure the distance of Betelgeuse, with proposed distances as high as 400 pc or about 1,300 ly.

Had they used the correct parallax and distance, would they have arrived at a diameter that would have been roughly correct?


texts:
I found an answer at
https://physics.stackexchange.com/questions/26795/how-to-calculate-the-amount-of-night-time-during-a-flight

but I was stuck at the part:

$\cosθ_1=\sinφ_1\sinφ+\cosφ_1\cosφ\cos(λ−λ_1)$

$\cosθ_2=\sinφ_2\sinφ+\cosφ_2\cosφ\cos(λ−λ_2)$

from which  $ (λ(t),φ(t))$

can be derived (after some tedious calculations).


texts: How did astronomers determine the path of 'Oumuamua so quickly?

texts:
The interstellar asteroid 'Oumuamua was discovered on October 19, 2017. Once discovered, less than a month's worth of precovery observations were found on various sky surveys. By mid-November, scientists had worked out its orbital eccentricity, and soon after its probable origin and destination.

With such a quick visit and so little data, how did scientists figure out so quickly where it came from and where it was going? Did using space probes for observations impart enough parallax to easily determine that, or did Earth-based observation have enough parallax themselves?


texts: What happened to the third planet in the Kepler-47 system?

texts:
In 2012, the discovery paper for the multiplanet circumbinary system Kepler-47 mentioned a potential third transiting planet in the system:

  A 0.2% deep transit-like event is present at time 2,455,977.363 (BJD) that is not caused by
  either of the two planets. A search for additional transits has revealed several more tentative
  transit events (12), but we caution that the star is faint (the Kepler magnitude is 15.178), there
  are large modulations due to star-spots, and the data contain correlated “red” noise, making
  small, non-periodic transit detection challenging. The marginal evidence at the present time is
  insufficient to place confidence on any additional candidate planet(s).

In 2013, Jerome Orosz gave a talk at the Second Kepler Science conference titled "The Confirmation of a Third Planet in the Kepler-47 Circumbinary System", claiming the detection of two further transits attributed to the third planet and giving the orbital period as 187.3 days and noting that the dynamical fit indicates that transits would stop for four years starting in July 2013.

In 2015, the discovery paper for Kepler-453 b was published, describing this planet as the tenth circumbinary planet discovered by the Kepler mission, which lists Kepler-47 d as the ninth of the previously discovered planets, giving a reference of "Orosz 2015, in preparation".

As far as I have been able to tell, there has been no further update since then - the Orosz 2015 paper never appeared. Is anyone aware of any further updates about this system and whether or not the third planet is still considered to be present?


texts: The Apollo missions took moving rovers to the Moon. How many moving rovers are presently on the Moon?


texts: Lets assume I have a source and receive $8 \times 10^{-20}$ ${\rm Wm}^{-2}$ in (400-750) nm filter. How do you convert this received flux to the unit Jansky, is there a formula? (Ultimately, I want to express this flux in magnitude AB.)


texts: Do liquids exist in the interstellar medium?

texts:
Am not asking about planetary/meteorite surfaces and am restricting myself to the interstellar medium

edit:
by interstellar medium I mean, the following:

  "Interstellar medium (ISM) is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space."

(copied verbatim from a google search).


texts: What is the theoretical ages for Pop I stars

texts: How old could population I stars theoretically be? What's the earliest they could have formed?


texts:
For example, if there was an object somewhere in the room, say 100 light years away, which would have to be very large and have the surface of a mirror, you could theoretically point the telescope at it and you would see the earth as it is was back then. Of course you would have to do all the details for years, the telescope would have to be much stronger and bigger, and something like this would have to exist and be discovered, but theoretically it could work that way. Your opinions would interest me very much!
Thank you


texts:
I'm wondering which telescope I should buy, by the way, I'm new to astronomy and if you know any others that are under $100, It'd be appreciated. You don't need to take the price difference into consideration when replying, thank you!
1 : http://www.dshop.com.au/buy/astronomical-telescope-675x-magnification/F90060M?gclid=CjwKCAjw-qbLBRB7EiwAftBCI7h_7lH3E9goJG4Top0N-OHaraxB0cM8IiZgfygRkHD8DZ1ThARCdRoCzA8QAvD_BwE
Features:
Maximum magnification: 675x
Large aperture offers clearly illuminated views
Stable construction provides solid support and control
Metal tripod with slow motion control rod
Strong performance in both high and low magnification
Easy sky focusing with finderscope
Specifications:
Aperture: 60mm (2.36inch)
Focal Length: 900mm.f/15
Finderscope:  5x24
Diagonal Prisms: 90degree
Slow Motion Control Rod For Easy Vertial Micro Adjustment
Height: 128cm
Eyepiece: SR4mm H12.5mm H20mm
3X Barlow Lens, 1.5X Erector
Telescopic Aluminum Tripod
2 : https://www.bhphotovideo.com/c/product/617033-REG/Celestron_21035_Travel_Scope_2_7_70mm_Refractor.html
Specifications:
70mm Achromatic Refractor
400mm Focal Length, f/5.7 Focal Ratio
Anti-Reflection Fully Coated Optics
1.25" Rack-and-Pinion Focuser
Manual Alt-Azimuth Mount with Lock
10mm and 20mm Eyepieces, Erecting Prism
5x24 Finderscope
Aluminum Photo-Style Tripod
Backpack Holds OTA, Tripod, Accessories


texts: How would a Fast Fourier Transform Telescope work without a mirror, dish or lenses?

texts:
I recently came across a short article mentioning the idea for a telescope built without mirrors or lenses that instead uses an array of antennas and massive computing power to resolve an image. Such a thing will apparently become possible if "Moore's law continues to hold", but doesn't explain how it will work.

Here's a link to the original paper: Tegmark and Zaldarriaga 2009

  We will show that it acts much like a single dish telescope with a
  dramatically larger field of view, yet is potentially much cheaper
  than a standard interferometer with comparable area. If a modern
  all-digital interferometer such as the MWA 1 is scaled up to a very
  large number of antennas N , its price becomes completely dominated by
  the computing hardware cost for performing of order N2 correlations
  between all its antenna pairs. The key idea behind the FFT Telescope
  is that, if the antennas are arranged on a rectangular grid, this cost
  can be cut to scale merely as N log2 N using Fast Fourier Transforms.
  As we will see, this design also eliminates the need for individual
  antennas that are pointable (mechanically or electronically),

Unlike a traditional interferometer, which consists of individual telescopes that focus light (or radio waves) to a point before ultimately combining their images, the antennas don't have a dish that focuses the radio waves from a certain point in the sky. Instead it's just an array of antennas that images the entire sky at once:

  The results above also show that the FFTT can be thought of as a cheap
  maximally compact interferometer with a full-sky primary beam.  To
  convert a state-of- the-art interferometers such as MWA 1, LOFAR
  [2], 12 PAPER[4], 21CMA [3] into an FFTT, one would need to do three
  things:

  Move all antenna tiles together so that they nearly touch.
  Get rid of any beamformer that “points” tiles to- wards a specific sky direction by adding relative phases to its component antennas, and
  treat each antenna as independent instead, thus allowing the array to
  image all sky directions simultaneously.
  Move the antennas onto a rectangular grid to cut the correlator cost from N2 to N log2 N .

Can someone explain how a computer can resolve an image without a lens, dish or mirror to make parallel rays of light converge on a sensor? I thought that a sensor without a lens, dish or mirror will just produce a blurry mess.

Edit: I think I understand it now. Watching this video was massively helpful for me in understanding how Fourier transformations allow you to isolate individual waveforms from a signal containing many different waveforms.


texts: What's the difference between apparent brightness and apparent magnitude? Are they the same?

texts:
I know that the equation for the difference in apparent magnitudes can be represented as:
$\Delta m=-2.5\log\left(\dfrac{I_{1}}{I_{2}}\right)$
and the apparent brightness of a celestial object is shown as:
$\ B=\dfrac{L}{4\pi\ D^2}$

Some people in my class told me that$\ B$ and$\ I$ were the same thing but used in different cases. This explanation didn't help me though. While playing around with the equations, I found this nice relation:
$\ \dfrac{B_{1}}{B_{2}}=\left[\dfrac {\left(\dfrac{L}{4\pi\ D_{2}^2}\right)}{\left(\dfrac{L}{4\pi\ D_{1}^2}\right)} \right]$
$\ \space\ \space\ \space\ ={\dfrac{4\pi\ D_{1}L}{4\pi\ D_{2}^2L}}$
$\ \space\ \space\ \space\ ={\dfrac{D_{1}^2}{D_{2}^2}}$

So if$\ {B_{1}}={I_{1}}$ and$\ B_{2}=I_{2}$holds true, is this following equation also true?

$\ {\dfrac{D_{1}^2}{D_{2}^2}} \space\ = 10^\left(-{\dfrac{\Delta {m}}{2.5}}\right)$
$\ \space\ \space\ \space\ \space\ =\dfrac{1}{10^\left({\dfrac{\Delta {m}}{2.5}}\right)}$

However, if$\ B$ and$\ I$ are not the same, what is the difference between the two measurements? And can either one be used to accurately measure how bright a non-luminous celestial body (ie. a planet) appears from Earth?

Also... does $\ m$ have an SI unit? I've never used any units when I solve for it


texts:
@csm's answer to Why not take a picture of a closer black hole? points out that it's necessary for the supermassive black hole at the center of a galaxy to be actively feeding for it to generate a radio-bright accretion disk that we can image. M87 is always feeding, but our own black hole only nibbles occasionally as a dust cloud passes by.
Question: How will they know when to start taking the picture of the black hole at the center of the Milky Way? Are there bits of food being tracked and they'll have all the EHT's telescopes ready when accretion begins? Or is it long enough that once it starts they can shuffle observing time around and still collect sufficient data?
For background see

What is it exactly about these flares of infrared light from Sgr A* that "confirms" it is a supermassive black hole?
How did they make a video of the center of the galaxy, and what is it exactly that's flashing there?

The ESA video ESOcast 173: First Successful Test of Einstein’s General Relativity Near Supermassive Black Hole includes a clip of images of stars at the center of our galaxy orbiting around SgrA*, a presumed supermassive black hole.
GIF made from video at around 02:50:

Six annotated frames from GIF highlighting the flashing that I'm seeing.


texts: What is the most asymmetric known planet?

texts:
Planets are more or less spherical, can you tell which one is the most asymmetric? Do they figure out such a property from an eccentric behaviour, or what?

I have a simple technical question: an asymmetric body gets a torque in a gravitational field, is the rotational energy/acceleration to be subtracted from the regular energy/acceleration? In other words: does an asymmetric planet get less radial acceleration than a perfect spherical one?


texts: What's the density of dark matter (clouds)?

texts: How much mass per unit volume do known dark matter have, for instance in the Bullet Cluster? Does the density vary in space, for example, do individual dark matter structures have denser cores?


texts: How do I calculate the inclination of an object with an amateur telescope?

texts:
Suppose I would like to calculate the inclination of a satellite from the ecliptic. Would it be possible to do this with an amateur telescope? How would I go about doing so?

Note: A good answer should tell what kind of telescope an amateur would need, what measurements they would need to make, then what calculations they would have to perform to get the inclination (or the "instantaneous angular measurement from the ecliptic at the time of measurement").


texts: What is a thermal distribution of eccentricities?

texts:
The abstract of the new (open access) paper in Nature A statistical solution to the chaotic, non-hierarchical three-body problem says:

  The three-body problem is arguably the oldest open question in astrophysics and has resisted a general analytic solution for centuries. Various implementations of perturbation theory provide solutions in portions of parameter space, but only where hierarchies of masses or separations exist. Numerical integrations show that bound, non-hierarchical triple systems of Newtonian point particles will almost always disintegrate into a single escaping star and a stable bound binary, but the chaotic nature of the three-body problem5 prevents the derivation of tractable6 analytic formulae that deterministically map initial conditions to final outcomes. Chaos, however, also motivates the assumption of ergodicity, implying that the distribution of outcomes is uniform across the accessible phase volume. Here we report a statistical solution to the non-hierarchical three-body problem that is derived using the ergodic hypothesis and that provides closed-form distributions of outcomes (for example, binary orbital elements) when given the conserved integrals of motion. We compare our outcome distributions to large ensembles of numerical three-body integrations and find good agreement, so long as we restrict ourselves to ‘resonant’ encounters (the roughly 50 per cent of scatterings that undergo chaotic evolution). In analysing our scattering experiments, we identify ‘scrambles’ (periods of time in which no pairwise binaries exist) as the key dynamical state that ergodicizes a non-hierarchical triple system. The generally super-thermal distributions of survivor binary eccentricity that we predict have notable applications to many astrophysical scenarios. For example, non-hierarchical triple systems produced dynamically in globular clusters are a primary formation channel for black-hole mergers, but the rates and properties of the resulting gravitational waves depend on the distribution of post-disintegration eccentricities.

If I understand correctly, the paper looks at scattering events where an incident third body interacts with an existing pair and after some interaction leaves with one member, the other being ejected. The eccentricity distribution is that of the surviving pair.

The results have important implications in astrophysics; so this question just focuses on one aspect.

Question: What exactly is a thermal distribution of eccentricities? Is there something like a temperature here somewhere and for a given value why would the expected distribution of eccentricities (for some large ensemble of initial states) be a straight line with $$\frac{d \sigma}{d \epsilon_B} = 2 \epsilon$$

Fig. 4 Marginal distributions of binary eccentricity and orientation: Click for full size

  a, c, e, dσ/deB against eccentricity eB. b, d, f, dσ/dCB against the cosine of the binary inclination, CB. Line styles represent ergodic outcome distributions with the same ensemble angular momenta as in Fig. 3. The data points are binned outcomes from the same numerical scattering ensembles as in Fig. 3, with each row corresponding to the same cuts on Nscram. The eccentricity outcome distributions are notably super-thermal (the thermal distribution, dσ/deB = 2e, is shown as a black dashed line). The inclination distributions exhibit anisotropic bias towards prograde binaries aligned with L0 (the isotropic distribution is shown with a black dashed line). Horizontal error bars show bin sizes and vertical error bars indicate 95% Poissonian confidence intervals.


texts: Can we conclude with reasonable certainty that there is/are no Type 3 Civilizations in our galaxy?

texts:
As per the Kardashev Scale, type-3 civilizations are defined as "civilization(s) in possession of energy on the scale of its own galaxy, with energy consumption at ~ 4 x 10^44 erg/sec". Considering that we have detected none, despite projects like SETI - can we conclude with reasonable certainty that there is "currently" no type-3 civilization in the milky way? Or Is it perhaps possible that there could be such civilization(s) primarily harnessing dark matter / energy and hence gone undetected?

I am assuming there could yet be type-2 civilizations that extract fusion energy ("normal/known" sources of energy), information, and raw-materials from multiple solar systems - within our own galaxy - yet undetected.


texts:
How is a galaxy formed? I know that the center of our galaxy is considered to be Sagittarius A* and it's surrounded by a lot of stars (also a lot of neutron stars).

But what makes this happen? I mean, why are all those stars in that right formation, like other galaxies, rather than just being random clouds of stars in the universe?


texts: Why can't ESA/NASA build a handful of Gaia telescopes and launch them into different positions in space for better accuracy?

texts: I've noticed that many stars in GAIA DR2 have highly uncertain distances. Why can't a space agency send multiple Gaia spacecraft into orbit around different places (One in L2 Earth, one in heliosynchronous orbit, and one around Jupiter) so the accuracy  of observations can be greatly improved?


texts: Is there any "scale" for measuring salinity of extraterrestrial water?

texts:
Continuation of Is the water underneath Europa's ice cap potable?

Recently, there was a question which discussed if water from Europa's ice caps is potable or not and from the answers, we gathered from the fact that the water is too salty to drink and only the most salt tolerant organisms would barely able to survive the environment.
Now there is a term called salinity which by definition is the saltiness or amount of salt dissolved in a body of water and is an important factor in determining many aspects of the chemistry of natural waters and of biological processes within it. With that term, there is a certain scale called Thalassic series often used to measure salt content of seawater and lakes/rivers of our planet, Earth and rate accordingly. Can we use such series for extraterrestrial water? Come to think of it, if we consider salinity a factor and create a certain scale, we can study many aspects like biological process and conditions and we can consider whether such conditions is suitable for sustaining life.
Question: Is there any "scale" to rate salinity of extraterrestrial water?


texts:
I read that the CMB were all the photons left free when the universe was cold enough for electrons got bound with protons (recombination) and the photons free in that moment continued traveling in the directions they were.

So I imagine:

the CMB as a sphere (with incredibly thick walls) increasing its radius continuously away from the 'center' of the universe. When the Milky Way formed, hadn't this sphere of photons already passed the space in which the Milky Way formed? How can we still see them if they are moving away from us.
the rate at which the space grew was bigger than the speed of these photons, so the Milky Way and our solar system had time to be formed before being reached by this photons lagged behind the speed in which the space increased.

On the other hand, how can we continuously measure the CMB if is not being generated continuously as a stream of photons like looking at a distant star. We should be able to see them only once when they passed by earth.


texts: Surface brightness of deep-sky objects and telescope specs

texts:
I'm new to amateur astronomy, and I recently acquired my first small telescope. I've just learned a bit about surface brightness after an unsuccessful night of trying to locate a few nebulae that were beyond my scope's abilities, but I'm wondering if it's as cut-and-dry as I've understood so far.

The reason I'm asking is that when I look up the surface brightness of the Great Orion Nebula on Stellarium, it shows up at 13.07, a bit beyond the limiting stellar magnitude of my scope at 11.7. However, I am pretty confident that I was able to see the Orion Nebula with my scope on another occasion. It's a large enough object that I'm pretty sure I observed it, and the features looked correct (but faint and colorless) when compared to high-detail photos.

So to get to the point of my question, are there other things I should keep in mind when determining whether I'll be able to see an object with my scope? I would like to avoid more frustrating nights in the future by not picking impossible targets. I also recognize that as an amateur I might have seen something completely different than the Orion Nebula.


texts: How common is it for large objects (few kilometers wide) in a young protoplanetary disk or debris disk to collide with one another?

texts:
If enough time has passed for gravitational forces to allow the formation of planetesimals(proto-planets) in orbit around the proto-star, how likely is it that two of them would collide with one another? If they do collide, would a head-on collision be more likely than a glancing collision?

What would be the observable effect of such a phenomena?


texts: What does the amino acid found in the atmosphere of Venus mean?

texts: An amino acid is a part of DNA, but it has only been found on Earth. Is the amino acid maybe from one of the space vehicles that has orbited, scanned, or surveyed Venus? How can a part of DNA survive in the dense, sulfuric acid-filled atmosphere? Scientist's haven't been able to determine what organism it's from, but it hasn't been brought to Earth yet. Could this be from the water that covered most of Venus billions of years ago? Did some type of life start to evolve before all the water evaporated? I'm kinda new to this site and have always wondered about this, so thanks to anyone who has an answer!


texts: Are space telescopes completely out of the earth's atmosphere?

texts: Is the Hubble Space Telescope, and every other space telescope for that matter, completely outside the Earths atmosphere?


texts: Eventual outcome of tidal acceleration and deceleration

texts:
So, I know the Moon experiences tidal acceleration from the Earth. And, from what I've read, if not for the fact that the sun would boil away the oceans and engulf both of them first, about 50 billion years from now the Moon would be geosynchronous.

I was curious whether tidal acceleration could ever cause a satellite to escape, and short answer seems to be, yes, it's possible. Is that accurate? And if so, which factors add to it?

A faster rotating planet also results in tides farther ahead, and has more energy to lose before being totally locked. Very closely related, a farther satellite orbits more slowly, resulting in tides farther ahead, and requires less energy to escape. This one seems obvious: the faster the planet is still rotating while the satellite is farther away, the more likely it escapes.
A more fluid planet would experience stronger tides, which would again slow it down faster, but definitely accelerate the satellite faster. A more viscous fluid than water would experience weaker tides, but I think they'd be farther ahead. I think more fluid is definitely more effective, but it seems less obvious than the first point.
A larger planet would have its orbit slowed less by tidal acceleration, but have a stronger pull to escape. A larger satellite would cause stronger tides and more tidal force, but would require more force to accelerate, and would slow the planet down more quickly. I really can't tell which has the stronger effect...

So that's question 1:
Is it really possible for tidal acceleration to throw a satellite into space before the planet is tidally locked, and if so, does either objects' size impact it, or just fluidity and relative periods?

Then, I wondered about tidal deceleration, and crashing into the planet. It seems obvious that any satellite orbiting retrograde would never stop decelerating, so all would eventually get torn apart and crash onto their planet. For non-retrogade satellites, an article I read implied that all of them will have the same fate... Which implies that the planet's rotation will always speed up more slowly than the satellite's falling orbit. Is that true? If not, then it seems there must be cases where the planet's rotation catches up, and they again end up tidally locked.

So that's question 2:
Will all non-retrogade tidally decelerating satellites eventually crash into their planet, or is it possible for the planet to catch up given the right starting conditions?

Edit: Haven't really found any more information on this. Is there someone familiar with relevant equations, who knows the answer?


texts: What are Kepler's laws (as he wrote them)?

texts:
There are of course many, many sources that quote Kepler's laws of planetary motion. This is preventing me from finding out what I really want to know: which is - what are Kepler's laws as he wrote them? As opposed to how they may have been later interpreted.

Very specifically I am interested in whether "Kepler's First Law", as proposed by Kepler, which is usually stated in the form that orbiting objects in the solar system execute elliptical orbits with the Sun at one focus, actually recognised, or had a caveat, that in a 2-body system it is the barycentre that is at the focus of an ellipse for both bodies, and that it is even more complex in the case of 3-bodies or more.


texts: How could a Star possibly be older than the Universe ? Is this possible or are the calculations off when determining the age of such Stars ? Can a Star exist that formed on its own independent of a Universe ?


texts: Mass of the stellar cluster (not the whole galaxy)

texts:
Is there any general formula to determine the total mass of a cluster of stars?
I have searched for one in Google, but most of them are for calculating mass of binary stars or single stars using Kepler's law. If the total mass can be found by multiplying the mass of a star by total number of stars, then how can the average mass of an individual star and the total number of stars in a cluster system be calculated? Will the equation vary for cluster system? Assume that the cluster is observable and  spectra are also attainable for the cluster.


texts: Can dark matter be found in the shape of planets, galaxies etc.?

texts:
If dark matter has gravity just like normal matter, does that mean it can also form planets, solar systems and so on?
 Any answer will be appreciated.


texts:
It is a said fact that 299792458 m/s is the speed limit,nothing can go faster than this speed, or it will take an infinite amount of energy for propulsion.What we perforce in vacuum is one thing.Can someone explain what will be observed(from the particle's frame and an observer frame at rest) if a particle of a non zero test mass moves at light speed inside a non vacuum medium(say water) as this velocity is definitely lesser than the limit.
THANKS IN ADVANCE


texts: Are there known objects rotating at close-to-light speeds?

texts:
For example, $0.1c < v < c$? Is this actually possible?

Background: is that true that the black hole in the center of the Milky Way rotates once per 20 minutes?


texts:
There is some discussion (and there are some questions on this site) about the impact of Starlink on observational astronomy. I am not an astronomer, but I am amazed by the beauty an immensity of the night sky.
Must I be concerned, from an aesthetical point of view, that Starlink will change the night sky that I and countless people before me have admired? For example, will there be less stars visible to the naked eye, or will the night sky be defaced by a myriad of artificial light sources?
For background: Starlink is a commercial project by SpaceX, a private company, that seeks to launch thousands of small satellites into the earth's orbit.


texts: How did Bradley arrive at the +/- correct speed of light when he calculated it?

texts:
After Bradley discovered stellar aberration and the corresponding constant of aberration he was able to calculate the speed of light, since he knew the speed of the Earth around the Sun.

As far as I think I understand the math behind his calculations. Yet, I believe that Bradley (and anyone else until Relativity) didn't consider the velocity of the Solar System itself. It seems (to me, that is) that Bradley assumed a Solar System at rest. If the Solar System is moving, this certainly affects the result of the calculation (especially so since at that time the speed of light was not considered constant).

I'd appreciate if someone pointed out to me where my thinking is wrong.


texts: Can Two Stars of One Same Type Orbit Each Other?

texts: This is a simple, textbook definition of a binary star orbit.  One low-mass star orbits a higher-mass star in a narrow but definitive center.  For example, the high-mass star would be our own sun--a G-type main sequence star--being orbited by a smaller dwarf, like a red dwarf or a brown dwarf.  But what about binary orbits of two identical stars--like our sun orbiting another G-type main-sequence star?  Is this sort of orbit possible?


texts: Question about the formation of elements

texts: So, elements such as iron, gold, etc. are formed from different types of supernovas, whether it's type 1A or 2. My question is, is it possible that there are elements that are formed that we never received during the formation of Earth and that we are unaware of? Or have we discovered every non man-made element?


texts: Is it possible to have a star orbiting around a brown dwarf?

texts: I know a single star is almost surely more massive than a single brown dwarf, but consider a case: a very small star is orbiting a very massive brown dwarf which has a very massive moon, such that the total mass of that brown dwarf and its moon is larger than that of the star. Not counting capturing, is this type of system possible (formed naturally)?


texts:
As far as I know, a brown dwarf is a 'star' whose core never underwent a fusion reaction, so it never became a star.

So I was wondering if, apart from orbiting a star, is there any difference between a massive planet and a brown dwarf?

I heard that they found an exoplanet 5 times the mass of Jupiter, and say a brown dwarf was in a binary pair where the other is a star: What stops it from being classified as a planet?

I guess what I am really asking for is a clearer definition of a brown dwarf.


texts: If the Earth was in Venus's orbit, would it be habitable?

texts:
If the Earth as-is were moved to Venus's orbit (and Venus removed), would the Earth remain habitable, would it eventually turn into what Venus currently is, or would it turn into something else?

I'm mainly wondering which characteristics of Venus (hot, no magnetic shielding, etc) are the result of how it is composed compared to it's proximity to the sun.

Similar question: If Venus and Mars changed places, would we then have 3 habitable planets?


texts: How to find inverse steradian from $\text{arcmin}^{-2}$ for density of galaxy

texts:
I am using a code on EUCLID future mission.
The original author of this code has set a value for the density of galaxy equal to :

ng = 354543085.80106884

I think this is expressed in inverse steradian. I think that EUCLID mission has a $30\text{ arcmin}^{-2}$ value for density of galaxies.

Is the conversion correct betweeen $30\text{ arcmin}^{-2}$ and $354543085.8010688 \text{ sr}^{-1}$ ?

and how to do this conversion ?

Indeed, I would like to calculate ng with a density of $48 \text{ arcmin}^{-2}$.

Regards


texts: I am aware that the vast majority of meteorites come from the asteroid belt (including 4 Vesta), with some from the Moon and Mars. Are there any other locations from which meteorites have originated (the Kuiper belt? Ceres?). Here it mentions that carbonaceous chondrites formed farthest from the Sun. Is that still within the asteroid belt?


texts:
I overheard a discussion between friends:

  If your spaceship was transported to a random location in our galaxy, you could identify nearby stars by their signature, and then triangulate your position.

Another friend replied.

  The signature of a star isn't sufficiently unique. You can't identify a star by spectroscopy.

A third friend chimed in

  There would be enough unique pulsars to work out your position from.

I came away not convinced on the stellar spectroscopy question.

My question is: Can you identify a star's 'signature' from its spectroscopy?


texts:
Big bang is not an explosion, but an expansion of space and time. Universe had almost infinite density... wait a moment. If it had ALMOST infinite density, it had a certain volume, and thus, space already existed.
Am I commiting a mistake?

I had this question after reading this post.


texts: Account for radiation energy in the statement "total energy of universe is zero"

texts: I've read in Hawking's book The Theory of Everything that total energy of the Universe is zero. He explained this by saying that if there are two bodies separated by a distance and having some mass, they possess potential energy and the same amount of negative energy  with the gravitational field. Thus he concluded that total energy of the Universe  is zero. But here I've a question: how do we account for energy which is in the form of radiation?


texts:
You've seen it in all kinds of science fiction, be it books, games, movies, and such. Space is depicted all sorts of ways, with asteroids far too close together, nebula thick enough to hide in and much more. Yet for a writer that want's to describe space accurately, this is a problem.

So I need stellar features and reasons why they would form/exist

Here's a rough list of what I'm talking about:

Thick asteroid style fields, averaging around 100 meters apart
Nebula regions that are actually thick enough to obscure light, like clouds
Dust cloud regions thick enough to obscure light
Any other kind of cloud of material that's similar to the two above
Naturally occurring fields of explosive gas or a physical material that does the same
Radiation levels high enough to eat through ship plating
Naturally occurring anomalies of any sort, such as black holes, rogue planets and stars

An example would be a protoplanetary disc. Could this allow for material in close proximity so you have rocks in orbit that are close together? What about clouds of material? Would a forming star would have a thick cloud of material around it?

Anything naturally occurring or even unnaturally occurring is fine, so long as space magic or whatever isn't involved.

If I missed anything you can think of, please elaborate. This topic has been difficult for me to find information on.

Extra question: Could a cloud or clouds of opaque gas exist, even for a short time in space? With or without other material present works.


texts:
I am trying to solve a problem where I need to calculate a satellite's orbit, but first I would like to ask for some clarifications from someone here that might know this stuff. I need to design an orbit so that no maneuvers (one part of the problem) are necessary to maintain it, and the hint is that there are J2 perturbation only and that the orbit is elliptical. I don't really understand this, so can someone explain what they mean with only J2 perturbations and how it is relevant here? (just asking for an explanation)

Thanks!